For the color-changing treatment of keratin fibers two coloring methods usually are used. In the first method, the coloration is produced with so-called oxidative or permanent colorants using a mixture of various developer substances and coupler substances and an oxidizing agent. If required, in this method, so-called direct (nonoxidative) dyes can be added to top off the coloring result or to produce particular color effects. The second method uses exclusively direct dyes, which are applied to the fibers in a suitable carrier mass. This method is easy to use, exceptionally gentle and is characterized by low damage to keratin fibers. The direct dyes used here are subject to a large number of requirements. For example, they have to be acceptable from a toxicological and dermatological point of view and allow the attainment of colorations in the desired intensity, which, inter alia, also requires adequate solubility in water. In addition, good lightfastness, acid fastness, and rubbing fastness is required for the colorations achieved.
Compared with oxidative colorations, nonoxidative colorations, however, generally have lower durability and a poorer eveness of color. In addition, direct colorants are generally not able to “lighten” the hair since many direct dyes do not withstand the oxidizing agents required for the lightening and/or the required pH of greater than or equal to 9.
WIPO Application No. WO 95/01722 A1 and WO 97/20545 A1 disclose colorants which comprise cationic azo dyes, in which the positive charge is delocalized in the conjugated system. Cationic azodyes, in which the cationic charge is localized in a side chain, are known to dye synthetic fibres, as described in European Patent No. EP 56578 A1, for example. German Patent No. DE 10118271 A1 discloses among others cationic disazodiazole derivatives and their use in hair dyes.